In 3GPP UMTS (Third Generation Partnership Project Universal Mobile Telecommunication System) wireless systems, the Random Access Channel (RACH) is an uplink (UL) transport channel that is used for transfer of data and/or control information in the absence of a dedicated radio link. The RACH is mapped to the physical random access channel (PRACH).
Access to the RACH by a wireless transmit-receive unit (WTRU) is based on a slotted-Aloha approach, with acquisition indication received from a radio access network (RAN). The WTRU must first acquire the channel by transmitting a preamble, which comprises a signature sequence that is randomly selected among a set of predetermined sequences. The transmit power of the initial preamble is determined by open loop power control, with parameters determined and broadcast by the RAN.
The WTRU then waits for an acquisition indication from a Node B, which is signaled in the Downlink (DL) on the Acquisition Indicator Channel (AICH). When the Node B detects the PRACH preamble associated with RACH attempt, it echoes on the AICH an identical signature sequence to indicate to the WTRU to transmit over PRACH.
In the case where no AICH is detected, the WTRU increases its transmission power by a predetermined amount and retransmits the preamble in the next available transmission slot. The process is repeated until the AICH is detected by the WTRU, or until a maximum number of preamble transmissions is reached. If a negative acknowledgement is received or the maximum number of transmissions is reached, RACH access has failed and a backoff procedure is performed at the medium access (MAC) layer.
In the case where a positive AICH is transmitted by the Node B, the WTRU transmits the PRACH frame, which consists of a control part 10 and data part 15 as shown in FIG. 1A.
The preamble and AICH procedure provide a way to for the WTRU to reserve the RACH as well as determine the right power for transmission. The power of the control part 10 is set with a fixed offset from the power of the last transmitted preamble. The transmission power of the data part 15 is set using a gain factor with respect to the control part, which is determined in the same way as other UL dedicated physical channels. The gain factor depends on the spreading factor that is used for the data part. Spreading factors 256, 128, 64 and 32 are allowed for the PRACH data part.
Referring to FIG. 2, the AICH consists of a sequence of consecutive access slots 20. Each access slot consists of two parts, an Acquisition-Indicator (AI) part 25 and a part 30 of duration 1024 chips with no transmission. The part of the slot with no transmission 30 is reserved for possible future use. The spreading factor (SF) used for channelization of the AICH is 256.
The transmission rate for RACH/PRACH is limited (single code with spreading factor 32) in existing 3GPP systems. One reason for the limitation is to avoid excessive UL interference caused by WTRUs when transmitting high rate bursts over RACH/PRACH. When a WTRU gains RACH access, it must independently select the transport format for transmission. There is no way for the RAN to dynamically control the transmission rate of WTRUs over RACH/PRACH.